1. Field of the Invention
The present invention relates to a process for making an abrasion resistant quasi monofilament having a high strength polyethylene fiber core with an adherent, abrasion resistant polymer sheath. More particularly, this invention relates to a process for making such a quasi monofilament by crosshead pressure extrusion sheathing at a temperature above the melting point of the core fibers.
2. Prior Art
Polyethylene yarns having very high tenacity have previously been described in U.S. Pat. Nos. 4,413,110, 4,430,383, 4,436,689, 4,536,536, and 4,545,950. The nature of the spinning processes used to prepare these yarns is such that individual filaments are typically of low denier and consequently break at low applied loads. To remedy this, multi-filament yarns may be used or several multi-filament yarns may be formed into a braid. However, such multi-filament yarns or braids are "limp", i.e., they have little flexural rigidity, and consequently bend easily when subjected to a transverse force. Additionally, in applications subjected to high abrasion, the individual low denier filaments of the multi-filament yarns may be rapidly worn through to produce debris or fibrillations which can snag or otherwise compromise the utility of the yarn or braid. In such applications it would therefore be highly desirable to have an article of high tenacity and high denier possessing high absolute breaking load together with flexural rigidity and high abrasion resistance. Examples of such applications are sports fishing lines, kite lines, dental floss, "weedeater" lines and tennis racket strings.
High strength polyethylene fibers or yarns coated with polyethylene or in a polyethylene matrix material are known. U.S. Pat. No. 4,501,856 discloses a composite of high strength polyethylene fibers in a matrix material which "may be any of the various known forms of polyethylene, including low density polyethylene (density of between 0.90 and 0.94 g/cm.sup.3), high density (between 0.94 and 0.98 g/cm.sup.3), linear low density or other conventional forms." U.S. Pat. Nos. 4,543,386 and 4,563,392 describe composites wherein high strength polyethylene fibers are coated on at least a portion of their surface with a solution of high density or low density polyethylene. U.S. Pat. Nos. 4,403,012 and 4,916,000 also mention coating high strength polyethylene fibers with polyethylene. JP-A-6022666 describes the preparation of fishing line which is composed of ultrahigh molecular weight polyethylene fibers which have been dipped in polyethylene wax at less than 90.degree. C.
The extrusion coating of aramid fiber bundles or ropes with polyethylene is also known. U.S. Pat. No. 4,202,164 describes a thermoplastic coated aramid rope wherein the coating can be polyethylene extruded under pressure into the interstices of the rope. U.S. Pat. No. 4,050,230 describes covering an uncured, thermosetting resin-coated bundle of aramid or glass fibers with molten polyethylene, for example by extrusion coating with the melt at a temperature of 200.degree. C. and under vacuum.
None of these publications, however, describes a process of crosshead pressure extrusion sheathing a high strength fiber core at a temperature above the melting point of the core fibers. These documents also fail to describe the use of a sheathing composition comprising as a critical component thereof a polymer having ethylene crystallinity, a density of at least 0.94 g/cm.sup.3, and a melt index of less than about 0.1 g/10 min.